Compound ejectors with improved off-design performance

Due to concerns over the environmental impact of refrigerants and the rising greenhouse gas emissions associated with building cooling, there has been a resurgence of interest in solar driven ejector cooling in recent times. Much of the research effort has concentrated on understanding the inner operations of the ejector using the approaches of thermodynamic modelling and computational fluid dynamics. Nevertheless several problems with ejector cooling persist. Amongst these are the constraints of constant cooling capacity at lower than design condensing temperatures and inability to operate at all above design condensing temperature remain. Compounding several ejectors in a series arrangement alleviates these two problems to some degree without requiring additional driving power, making the compound ejector design particularly suitable for dry re-cooling applications where condensing temperatures are elevated. A modelling study was conducted for a two stage compound ejector arrangement. The study concludes that about 1/3 of the solar energy should go to the first (low pressure, upstream) ejector and 2/3 to the second ejector for best cooling yield. Furthermore, the mixing chambers of each ejector can have the same geometry which affords some economies of production. The main outcomes of the study suggest that several persistent complaints over ejector operational characteristics and off-design performance can be alleviated using the compounding approach. Further work is warranted to determine the optimum number of compounding stages and degree of intercooling between stages.